CHEM J-3 June 2012

Transcription

1 CEM J-3 June 2012 In a standard acid-base titration, ml of M a solution was found to react exactly with ml of an Cl solution of unknown concentration. What is the p of the unknown Cl solution at 25 C? 3 The reaction follows the equation + (aq) + - (aq) à 2 (l) so that the number of moles of + that reacts is equal to the number of moles of -. For - : number of moles = concentration volume = mol L L = mol This is equal to the number of moles of + (aq) in ml, so: concentration of + (aq) = number of moles / volume = mol / L = mol L -1 Using p = -log 10 [ + (aq)]: p = -log 10 (0.9165) = 1.04 p = 1.04

2 CEM J-4 June 2010 Glycine, 2 C 2 C, is the simplest of all naturally occurring amino acids. The pk a of the acid group is 2.35 and the pk a associated with the amino group is Draw a structure that indicates the charges on the molecule at the physiological p of This p is much greater than the pk a value of the acid group: it is deprotonated. This p is much lower than the pk a value of the amino group: it is protonated. Use your structure to illustrate the concept of resonance. What are the hybridisation states and geometries of the two carbon atoms and the nitrogen atom in glycine? The carbon on the acid group is sp 2 hybridised and the geometry is trigonal planar. The carbon on the C 2 group is sp 3 hybridised and the geometry is tetrahedral. The nitrogen is sp 3 hybridised and the geometry is tetrahedral. Propionic acid, C 3 C 2 C, has a melting point of 20.7 C while glycine has a melting point of 292 C. Suggest a reason why these two molecules have such different melting points. Propionic acid has strong hydrogen bonds, giving it a relatively high melting point. owever, glycine has very strong ionic bonds between the 3 + and C 2 groups giving it very high melting point.

3 CEM J-5 June 2010 Explain the terms weak and strong and the terms dilute and concentrated in the context of acids and bases. 2 A weak acid or base is one which only partially dissociates in water: e.g. C 3 C(aq) C 3 C 2 (aq) + + (aq) A strong acid or base is one which completely dissociates in water: e.g. Cl(aq) + (aq) + Cl (aq) Concentrated and dilute are terms that can be used in reference to any solute, describing the number of moles of solute relative to the volume of solvent. A concentrated solution has a high solute : solvent ratio, whilst a dilute solution has a low solute:solvent ratio.

4 CEM J-3 June 2009 The molecular structure of adrenaline (epinephrine), a hormone involved in the "fight or flight" response, is shown below. 4 A B #1 #3 C #2 List the types of intermolecular interactions that each of the following sites on adrenaline would be involved in if dissolved in water. A -bonding, dipole-dipole, dispersion forces B dispersion forces C -bonding, dipole-dipole, dispersion forces Pharmaceuticals with amine groups are frequently supplied as their hydrochloride salts. Draw the structure that would result if adrenaline were reacted with one equivalent of Cl. What additional intermolecular forces would be present if this form of adrenaline were dissolved in water? Cl With this form present, ion-dipole interactions would be introduced.

5 CEM J-3 June 2008 The pk b of -1 is and the pk b of -2 is Draw the structure of the predominant form of nicotine that exists in the human body at p For -1, the pk a of the protonated form (the conjugate acid) is ( ) = As the p is higher than the pk a, the conjugate acid is deprotonated: very little protonation occurs. For =2, the pk a of the protonated form is ( ) = As the p is lower than the pk a, the conjugate acid form dominates: protonation occurs.

6 CEM J-4 June 2008 Lithium salts, especially lithium carbonate, are commonly used in the treatment of bipolar disorder. Write the net ionic equation for the reaction which occurs between lithium carbonate and hydrochloric acid in the stomach. 4 Li 2 C 3 (s) (aq) 2Li + (aq) + 2 (l) + C 2 (g) Lithium orotate (as a monohydrate salt, LiC ) is a controversial alternative formulation sold in some health food stores. The orotate ion is the conjugate base of orotic acid, whose structure is shown below. Like the carbonate, lithium orotate is taken orally. Using an equation, comment on any differences between the form in which lithium is bioavailable from these two lithium salts. orotic acid When lithium orotate, LiC , dissolves in water, it forms Li + (aq) ions and orotate ions: LiC (s) Li + (aq) + C (aq) Both lithium carbonate and lithium orotate thus give rise to the same form of lithium, Li + (aq), when taken orally.

7 CEM J-3 June 2007 Glycine, 2 C 2 C, the simplest of all naturally occurring amino acids, has a melting point of 292 C. The pk a of the acid group is 2.35 and the pk a associated with the amino group is Draw a structure that indicates the charges on the molecule at the physiological p of As p = 7.4 is higher than the pk a of the acid group, -C, it will exist primarily in its deprotonated, conjugate base form, -C -. As p = 7.4 is lower than the pk a of the amino group, - 2, it will exist primarility in its protonated form, Glycine will exist in the uncharged, zwitterionic form: 3 C 2 C Describe the hybridisation of the two carbon atoms and the nitrogen atom in glycine and the geometry of the atoms surrounding these three atoms. The structure is: has 4 bonds and no lone pairs: sp 3 with a tetrahedral arrangement. C a C b C a has 4 bonds and no lone pairs: sp 3 with a tetrahedral arrangement. C b has 3 bonds and no lone pairs: sp 2 with a trigonal planar arrangement. Glycine has an unusually high melting point for a small molecule. Suggest a reason for this. Glycine with a positively and a negatively charged end. There is therefore ionic bonding between the molecules leading to strong intermolecular forces.

8 CEM J-5 June 2006 Consider the following equation. Br(aq) + 3 (aq) Br (aq) (aq) ame all of the species in this equation. 5 Br Br hypobromous acid hypobromite ion ammonia ammonium ion Complete the following table by giving the correct pk a or pk b value where it can be calculated. Mark with a cross ( ) those cells for which insufficient data have been given to calculate a value. Species Br 3 Br 4 + pk a of acid pk b of base Determine on which side (left or right hand side) the equilibrium for the reaction above will lie. Provide a brief rationale for your answer. The reaction is the sum of the acid-base equilibra for Br and 3 : Br(aq) + (aq) + Br (aq) K a (Br) = (aq) + 3 (aq) + 4 (aq) K( 3 ) = + K ( ) a 4 Br(aq) + 3 (aq) Br (aq) K = K a (Br) K( 3 ) = ence, K = ( ) ( ) = = 4.4. As K > 1, the reaction favours products.